1. Field of the Invention
The present invention relates to an optical data reading method using a reading apparatus for an optical memory (referred to as an "optical pickup", hereinafter), and in particular to an optical data reading method and an optical data reading apparatus suitably used for a high density optical recording system.
2. Description of the Related Art
An optical memory system utilizing light for recording reproducing data is widely used today due to advantages thereof such as large capacity, lower cost per bit, and portableness. So, such an optical system is put into practical use as read only memories such as compact discs and video discs and re-writable optical memories such as magneto-optical discs. In the future, such optical memories having a larger capacity and a smaller size will be demanded as society becomes more and more information-oriented.
FIG. 20 shows one configuration of a conventional optical pickup designed for reading data stored in a magnetooptical disc. This optical pickup includes a semiconductor laser element 1901 serving as a light source, a collimator lens 1902, a beam shaping prism 1903, a first beam splitter 1904, an objective lens 1905, a magnetooptical recording medium 1906, a second beam splitter 1907, a .lambda./2 plate 1908, a polarization beam splitter 1909, signal light condenser lenses 1910 and 1911, PIN light receiving elements 1912 and 1913, a condenser lens 1915 for detecting a tracking error/focus error, a light receiving element 1916 for detecting a tracking error/focus error, and a magnet 1917 for applying a magnetic field to the magnetooptical recording medium (hereinafter, referred to as a "disc") 1906. The operation of this optical pickup will be described hereinafter.
A laser beam emitted diagonally upwards by the semiconductor laser element 1901 is converted into a plane wave by the collimator lens 1902 and is incident onto the beam shaping prism 1903. The laser beam incident onto the beam shaping prism 1903 is shaped into a circular beam. Herein, the light emitted by the semiconductor laser element 1901, passed through the collimator lens 1902 and output from the beam shaping prism 1903 is all linearly polarized light.
Then, the linearly polarized laser light is condensed by the objective lens 1905, and then radiated onto the magnetooptical disc 1906 located above the objective lens 1905. Data is digitally recorded in the magnetooptical disc 1906 by magnetizing the magnetooptical disc 1906 perpendicularly to the surfaces thereof. When the laser beam is reflected by the magnetoopical disc 1906, the plane of polarization of the laser beam is rotated by a Kerr effect in accordance with the data stored in the magnetooptical disc 1906. Depending on whether the recorded digital data (digital signal) is "0" or "1", the rotation direction of the plane of polarization of the laser beam when the data is "1" is opposite to that when the data is "0". Accordingly, using this principle, the digital data "0" and "1" can be read by detecting in which direction the plane of polarization is rotated.
The signal light reflected by the magnetooptical disc 1906 is turned at 90.degree. by the first beam splitter 1904 and then incident onto the second beam splitter 1907, whereby it is divided into first and second light components in horizontal and vertical directions, respectively. The first component in a horizontal direction serves as a beam for detecting a tracking error/focus error. The first light component is condensed by the condenser lens 1915, and thereafter is guided to the light receiving element 1916 so as to be photoelectrically converted. Thus, a tracking error/focus error signal, which is an electric signal, is obtained.
On the other hand, the second light component in a vertical direction serves as light for detecting a recorded signal. The plane of polarization of the second light component is rotated at 45.degree. by the .lambda./2 plate 1908. FIG. 21 shows polarization components of reflection light from the disc 1906 in this state. At this time, within the reflection light from the disc 1906, light 2001 of which plane of polarization is rotated by the disc 1906 and a non-rotation component 2002 resulting from the reflection from surfaces and the like of various optical components located in the optical path, are superimposed on each other. Then, by the polarization beam splitter 1909 located on the outgoing side of the .lambda./2 plate 1908, the reflection light including thus superimposed components is divided into two components perpendicular to each other, i.e., an s-wave component and a p-wave component as shown in FIG. 21.
The p-wave component is transmitted straight through the polarization beam splitter 1909, i.e., the beam travels downwards in a vertical direction, whereas the s-wave component is turned at 90.degree. by the polarization beam splitter 1909 so as to travel in a horizontal direction. Following this, the p-wave component is condensed by the condenser lens 1911 and guided to the high speed PIN light receiving element 1913 so as to be photoelectrically converted. Likewise, the s-wave component is condensed by the condenser lens 1910 and thereafter guided to the high-speed light receiving element 1912 so as to be photoelectrically converted. The electric signals respectively converted by the two high speed PIN light receiving elements 1912 and 1913 are amplified by the differential amplifier 1914 to a predetermined level, and any difference therebetween is detected. Hence, the non-rotation reflection light component 2002 as shown in FIG. 21 is eliminated. As a result, only the signal light 2001 which is the light component of the light rotated by the recording magnetization is detected as an electric signal. Then, the recorded data is reproduced from a detection output of the differential amplifier 1914.
In the case of the signal light 2001 shown in FIG. 21, the p-wave component is larger than the s-wave component. Accordingly, the detection output from the differential amplifier 1914, i.e., (p-wave component)-(s-wave component) is an electric signal having a positive value. Conversely, in the case where the plane of polarization is rotated in the opposite direction to that of the signal light 2001, i.e., in the case where the optical data is recorded in the magnetooptical disc 1906 by magnetizing the disc in the opposite direction, the s-wave component is larger than the p-wave component. In such a case, the detection output is an electric signal having a negative value.
In the above mentioned optical pickup, through the differential amplifier 1914 of a type directly detecting the intensity of light is provided as a means for detecting the recorded data, an optical pickup provided with a detection means employing a direct detection method of other types is also known. However, in all types of conventional optical pickup including an optical pickup for a magnetooptical disc, a direct detecting method for directly detecting the intensity of light is used for signal detection.
For an optical disc memory, a larger capacity and a higher reading speed (realization of a high-speed access) will be increasingly demanded. In order to fulfill such demands, a magnetooptical disc having a higher density recording medium and a higher rotation speed is required. In accordance with such development in the density and the optation speed, a recording area allocated for one bit. i.e., one recording unit is further reduced. As a result, the intensity of signal light reflected by the recording medium is lowered, and the pulse width of the signal light per bit is decreased. In other words, the amount of energy of the signal light per bit is reduced.
Under these circumstances, in an optical pickup having the above-mentioned light intensity detecting mechanism, the power level of the signal light becomes close to those of shot noises and thermal noises in the light receiving circuit. For this reason, an existing optical pickup system using light having a wavelength of 780 nm has a problem in that the bit error rate exceeds 10.sup.-5 when reading data stored in a disc having a high density of 1 Mbit/mm.sup.2 or more, which is disadvantageous in practical use. This is because, in the direct detection method, a C/N value is degraded in proportion to the fourth power of the attenuation of the intensity of the reflection light, and in inverse proportion to the first power of an increase in the data reading rate.
As one method for solving this problem, the use of laser beams having a shorter wavelength as a light source has been actively studied. However, a laser of a short wavelength is not put into practical use in accordance with the needs for such an optical disc memory with a large capacity. Even if a laser for emitting light having a desirably short wavelength is developed, an optical memory having a still larger capacity which can be operated using such a laser is demanded. In consideration of these matters, this approach also has a problem in that a highly sensitive detecting mechanism is required.
Also, for realizing the recording of data at a higher density, a method of improving the substantial recording density by configuring the recording medium so as to be multi-layered has been examined. However, in a method of reading data from a conventional multi-layered recording medium, for selecting a specific recording medium layer within the multi-layered recording medium, there has been adopted an element using only the focal depth of a lens for condensing and radiating a laser beam. As a result, the reflection light is likely to overlap with the data from the layers other than a reading target layer, which causes noise. Thus, the data reading at a sufficient bit error rate cannot be achieved.
For the above-mentioned reasons, a conventional optical pickup cannot be used with improved optical discs of a larger capacity and higher recording density and is capable of accessing at a higher speed.